Are modern sleep patterns a product of recent selective pressures, or a mismatch to ancestral sleep biology? This is a question that has baffled and obsessed circadian researchers for some time. Samson et al. describes the circadian patterns in 33 East African Hadza hunter-gatherers over a total of 393 days in a paper published in January of 2017 in the American Journal of Physical Anthropology.
These recordings were compared with previous studies that detail the sleep-wake patterns and habits of post-industrialised Westerners. The study provides an important segregation of those following what might be called a modern pattern of living, and those that live in a “pre-industrial” context.
Sleep patterns in this group were tracked with wrist actigraphy devices. With these, sleep-wake patterns in thirty-three volunteers for a total of 393 days were tracked. Linear missed effects, and functional linear modelling were used to characterise and predict ecological predictors of sleep duration and quality, and averaged circadian patterns. Electroencephalogram worn at night for nine villagers over nine nights were used to analyse sleep further.
To put the results into context, two zeitgeber must be known; light and temperature. Light influences circadian timing, via the suprachiasmatic nucleus, and temperature, via cold and warm sensing neurons.
Hadza sleep was responsive – environmental factors defined sleeping patterns. Hadza people slept less at night despite longer durations of time in bed, and has a longer sleep latency and greater wake after sleep onset (WASO). Additionally, this population has lower sleep efficiency. Daytime napping in the hottest periods of the day compensated for the shorter sleep times overall. Circadian rhythms are characterised by greater intensity of activity during their active periods. The Hadza have a distinctly segmented sleep-wake pattern, characterised by a primarily monophasic and short nocturnal sleep bout supplemented by daytime napping.
This study demonstrates that circadian rhythms are not simply a function of photoperiod or temperature variations – not artificial changes in lighting or temperature. That natural sleep has the propensity for timing to be expressed in response to different ecological, social, and technological factors influencing sleep. The economic system and subsistence strategy that makes demands on activity at specific times during the circadian period to a certain extent define sleep period.
This, in combination with other research indicates a high degree of physiological plasticity; sleep patterns in populations with highly divergent environmental contexts conform. Further, that our sleep is defined by zeitgeber. What this means in a modern context is unclear – is sleep able to adapt to the bright, temperature controlled reality of today, or have we pushed things too far, beyond the reach of sleep plasticity? If not, what does that sleep look like?
Reference
Samson DR, Crittenden AN, Mabulla IA, Mabulla AZP, and Nunn CL. Hadza sleep biology: Evidence for flexible sleep-wake patterns in hunter-gatherers. Am J Phys Anthropol. 2017;162:573–582. doi:10.1002/ajpa.23160.
